Selective production of para-xylene

ABSTRACT

Process for the selective production of para-xylene by methylation of toluene in the presence of a catalyst comprising a crystalline aluminosilicate zeolite, said zeolite having a silica to alumina ratio of at least about 12 and a constraint index, as hereinafter defined, within the approximate range of 1 to 12, which catalyst has undergone prior modification by the addition thereto of phosphorus oxide and magnesium oxide, each in an amount of at least about 0.25 percent by weight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the selective production ofpara-xylene by catalytic production of para-xylene in the presence of aphosphorous and magnesium-containing crystalline aluminosilicate zeolitecatalyst.

2. Description of the Prior Art

Alkylation of aromatic hydrocarbons utilizing crystallinealuminosilicate catalysts has heretofore been described. U.S. Pat. No.2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons withan olefin in the presence of a crystalline metallic aluminosilicatehaving uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat.No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons inthe presence of X- or Y-type crystalline aluminosilicate zeolites,specifically such type zeolites wherein the cation is rare earth and/orhydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No.3,751,506 to Burress describe vapor phase alkylation of aromatichydrocarbons with olefins, e.g., benzene with ethylene, in the presenceof a ZSM-5 type zeolite catalyst.

The alkylation of toluene with methanol in the presence of a cationexchanged zeolite Y has been described by Yashima et al. in the Journalof Catalysis 16, 273-280 (1970). These workers reported selectiveproduction of para-xylene over the approximate temperature range of 200°C., with the maximum yield of para-xylene in the mixture of xylenes,i.e., about 50 percent of the xylene product mixture, being observed at225° C. Higher temperatures were reported to result in an increase inthe yield of meta-xylene and a decrease in production of para- andortho-xylenes.

U.S. Pat. No. 3,965,208 describes the methylation of toluene, underconditions such that the formation of meta-xylene is suppressed and theformation of ortho- and para-xylene is enhanced carried out in thepresence of a catalyst comprising a crystalline aluminosilicate zeoliteof the ZSM-5 type which has been modified by the addition thereto of asmall amount of a Group VA element. My previous patent, U.S. Pat. No.4,011,276, describes the disproportionation of toluene by subjecting thesame to disproportionation conditions in the presence of a catalystcomprising a crystalline aluminosilicate zeolite of the ZSM-5 type whichhas been modified by the addition thereto of a minor proportion of anoxide of phosphorus and a minor proportion of an oxide of magnesium toproduce benzene and xylenes rich in the para isomer.

While the above noted prior art is considered of interest in connectionwith the subject matter of the present invention, the methylationprocess described herein utilizing a catalyst of a crystallinealuminosilicate zeolite having a silica/alumina ratio of at least about12, a constraint index within the approximate range of 1 to 12 and whichhas been modified by the addition thereto of a minor proportion of anoxide of phosphorus and a minor proportion of an oxide of magnesium tothereby achieve unexpectedly high selective production of para-xylenehas not, insofar as is known, been heretofore described.

Of the xylene isomers, e.g., ortho-, meta- and para-xylene, the latteris of particular value being useful in the manufacture of terephthalicacid which is an intermediate in the manufacture of synthetic fiberssuch as "Dacron". Mixtures of xylene isomers either alone or in furtheradmixture with ethylbenzene, generally containing a concentration ofabout 24 weight percent para-xylene in the equilibrium mixture, havebeen previously separated by expensive superfraction and multistagerefrigeration steps. Such process, as will be realized, has involvedhigh operation costs and has a limited yield.

SUMMARY OF THE INVENTION

In accordance with the present invention, there has been discovered aprocess for selectively producing para-xylene in preference tometa-xylene or ortho-xylene by reaction of toluene with a methylatingagent in the presence of a catalyst comprising a crystallinealuminosilicate zeolite, said zeolite having a silica to alumina ratioof at least about 12 and a constraint index of from 1 to 12, whichcatalyst has been modified by the addition thereto of a minor proportionof an oxide of phosphorus and a minor proportion of an oxide ofmagnesium.

Compared to a conventional thermodynamic equilibrium xylene mixture inwhich the para:meta:ortho ratio is approximately 1:2:1, the processdescribed herein affords a xylene product in which the para-xylenecontent may exceed 90 percent. The improved yields of para-xylenereduces the cost of separation of para-xylene from its isomer which isthe most expensive step in the current method employed for producingpara-xylene.

The present process comprises methylation of toluene in the presence ofa catalyst comprising a particular crystalline aluminosilicate zeolitemodified by the addition thereto of phosphorus oxide and magnesiumoxide, each in an amount of at least about 0.25 percent by weight.

The crystalline aluminosilicate zeolite employed is a member of a novelclass of zeolites exhibiting some unusual properties. These zeolitesinduce profound transformation of aliphatic hydrocarbons to aromatichydrocarbons in commercially desirable yields and are generally highlyeffective in conversion reactions involving aromatic hydrocarbons.Although they have unusually low alumina contents, i.e., high silica toalumina ratios, they are very active even when the silica to aluminaratio exceeds 30. The activity is surprising since catalytic activity isgenerally attributed to framework aluminum atoms and cations associatedwith these aluminum atoms. These zeolites retain their crystallinity forlong periods in spite of the presence of steam at high temperature whichinduces irreversible collapse of the framework of other zeolites, e.g.,of the X and A type.

An important characteristic of the crystal structure of this class ofzeolites is that it provides constrained access to, and egress from theintracrystalline free space by virtue of having a pore dimension greaterthan about 5 Angstroms and pore windows of about a size such as would beprovided by 10-membered rings of oxygen atoms. It is to be understood,of course, that these rings are those formed by the regular dispositionof the tetrahedra making up the anionic framework of the crystallinealuminosilicate, the oxygen atoms themselves being bonded to the siliconor aluminum atoms at the centers of the tetrahedra. Briefly, thepreferred type zeolites useful in this invention possess, incombination: a silica to alumina mole ratio of at least about 12; and astructure providing constrained access to the crystalline free space.

The silica to alumina ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although zeolites with a silica to aluminaratio of at least 12 are useful, it is preferred to use zeolites havinghigher ratios of at least about 30. Such zeolites, after activation,acquire an intracrystalline sorption capacity for normal hexane which isgreater than that for water, i.e., they exhibit "hydrophobic"properties. It is believed that this hydrophobic character isadvantageous in the present invention.

The type zeolites useful in this invention freely sorb normal hexane andhave a pore dimension greater than about 5 Angstoms. In addition, thestructure must provide constrained access to larger molecules. It issometimes possible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of oxygen atoms, then access bymolecules of larger cross-section than normal hexane is excluded and thezeolite is not of the desired type. Windows of 10-membered rings arepreferred, although, in some instances, excessive puckering or poreblockage may render these zeolites ineffective. Twelve-membered rings donot generally appear to offer sufficient constraint to produce theadvantageous conversions although puckered structures exist such as TMAoffretite which is a known effective zeolite. Also, structures can beconceived, due to pore blockage or other cause, that may be operative.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access, a simpledetermination of the "constaint index" may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a small sample, approximately 1 gram or less, ofcatalyst at atmospheric pressure according to the following procedure. Asample of the zeolite, in the form of pellets or extrudate, is crushedto a particle size about that of coarse sand and mounted in a glasstube. Prior to testing, the zeolite is treated with a stream of air at1000° F. for at least 15 minutes. The zeolite is then flushed withhelium and the temperature adjusted between 550° F. and 950° F. to givean overall conversion between 10% and 60%. The mixture of hydrocarbonsis passed at 1 liquid hourly space velocity (i.e., 1 volume of liquidhydrocarbon per volume of zeolite per hour) over the zeolite with ahelium dilution to give a helium to total hydrocarbon mole ratio of 4:1.After 20 minutes on stream, a sample of the effluent is taken andanalyzed, most conveniently by gas chromatography, to determine thefraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Zeolites suitable for the presentinvention are those having a constraint index in the approximate rangeof 1 to 12. Constraint Index (CI) values for some typical zeolites are:

    ______________________________________                                        CAS                      C.I.                                                 ______________________________________                                        ZSM-5                    8.3                                                  ZSM-11                   8.7                                                  ZSM-12                   2                                                    ZSM-38                   2                                                    ZSM-35                   4.5                                                  TMA Offretite            3.7                                                  Beta                     0.6                                                  ZSM-4                    0.5                                                  H-Zeolon                 0.4                                                  REY                      0.4                                                  Amorphous Silica-Alumina 0.6                                                  Erionite                 38                                                   ______________________________________                                    

It is to be realized that the above constraint index values typicallycharacterize the specified zeolites but that such are the cumulativeresult of several variables used in determination and calculationthereof. Thus, for a given zeolite depending on the temperature employedwithin the aforenoted range of 550° F. to 950° F., with accompanyingconversion between 10% and 60%, the constraint index may vary within theindicated approximate range of 1 to 12. Likewise, other variables suchas the crystal size of the zeolite, the presence of possible occludedcontaminants and binders intimately combined with the zeolite may affectthe constraint index. It will accordingly be understood by those skilledin the art that the constraint index, as utilized herein, whileaffording a highly useful means for characterizing the zeolites ofinterest is approximate, taking into consideration the manner of itsdetermination, with probability, in some instances, of compoundingvariable extremes. However, in all instances, at a temperature withinthe above-specified range of 550° F. to 950° F., the constraint indexwill have a value for any given zeolite of interest herein with theapproximate range of 1 to 12.

The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11,ZSM-12, ZSM-35 and ZSM-38 and other similar materials. U.S. Pat. No.3,702,886 describing and claiming ZSM-5 is incorporated herein byreference.

ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, theentire contents of which are incorporated herein by reference.

ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449, theentire contents of which is incorporated herein by reference.

ZSM-35 is more particularly described in U.S. Pat. No. 4,016,245, theentire contents of which is incorporated herein by reference.

ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859, theentire contents of which is incorporated herein by reference.

The specific zeolites described, when prepared in the presence oforganic cations, are catalytically inactive, possibly because theintracrystalline free space is occupied by organic cations from theforming solution. They may be activated by heating in an inertatmosphere at 1000° F. for one hour, for example, followed by baseexchange with ammonium salts followed by calcination at 1000° F. in air.The presence of organic cations in the forming solution may not beabsolutely essential to the formation of this type zeolite; however, thepresence of these cations does appear to favor the formation of thisspecial type of zeolite. More generally, it is desirable to activatethis type catalyst by base exchange with ammonium salts followed bycalcination in air at about 1000° F. for from about 15 minutes to about24 hours.

Natural zeolites may sometimes be converted to this type zeolitecatalyst by various activation procedures and other treatments such asbase exchange, steaming, alumina extraction and calcination, incombinations. Natural minerals which may be so treated includeferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite,and clinoptilolite. The preferred crystalline aluminosilicates areZSM-5, ZSM-11, ZSM-12, ZSM-38 and ZSM-35 with ZSM-5 particularlypreferred.

In a preferred aspect of this invention, the zeolites hereof areselected as those having a crystal framework density, in the dryhydrogen form, of not substantially below about 1.6 grams per cubiccentimeter. It has been found that zeolites which satisfy all three ofthese criteria are most desired because they tend to maximize theproduction of gasoline boiling range hydrocarbon products. Therefore,the preferred zeolites of this invention are those having a constraintindex as defined above of about 1 to about 12, a silica to alumina ratioof at least about 12 and a dried crystal density of not less than about1.6 grams per cubic centimeter. The dry density for known structures maybe calculated from the number of silicon plus aluminum atoms per 1000cubic Angstroms, as given, e.g., on Page 19 of the article on ZeoliteStructure by W. M. Meier. This paper, the entire contents of which areincorporated herein by reference, is included in "Proceedings of theConference on Molecular Sieves, London, April 1967," published by theSociety of Chemical Industry, London, 1968. When the crystal structureis unknown, the crystal framework density may be determined by classicalpyknometer techniques. For example, it may be determined by immersingthe dry hydrogen form of the zeolite in an organic solvent which is notsorbed by the crystal. It is possible that the unusual sustainedactivity and stability of this class of zeolites is associated with itshigh crystal anionic framework density of not less than about 1.6 gramsper cubic centimeter. This high density, of course, must be associatedwith a relatively small amount of free space within the crystal, whichmight be expected to result in more stable structures. This free space,however, is important as the locus of catalytic activity.

Crystal framework densities of some typical zeolites are:

    ______________________________________                                                       Void         Framework                                         Zeolite        Volume       Density                                           ______________________________________                                        Ferrierite     0.28   cc/cc     1.76 g/cc                                     Mordenite      .28              1.7                                           ZSM-5, -11     .29              1.79                                          Dachiardite    .32              1.72                                          L              .32              1.61                                          Clinoptilolite .34              1.71                                          Laumontite     .34              1.77                                          ZSM-4 (Omega)  .38              1.65                                          Heulandite     .39              1.69                                          P              .41              1.57                                          Offretite      .40              1.55                                          Levynite       .40              1.54                                          Erionite       .35              1.51                                          Gmelinite      .44              1.46                                          Chabazite      .47              1.45                                          A              .5               1.3                                           Y              .48              1.27                                          ______________________________________                                    

When synthesized in the alkali metal form, the zeolite is convenientlyconverted to the hydrogen form, generally by intermediate formation ofthe ammonium form as a result of ammonium ion exchange and calcinationof the ammonium form to yield the hydrogen form. In addition to thehydrogen form, other forms of the zeolite wherein the original alkalimetal has been reduced to less than about 1.5 percent by weight may beused. Thus, the original alkali metal of the zeolite may be replaced byion exchange with other suitable ions of Groups IB to VIII of thePeriodic Table, including, by way of example, nickel, copper, zinc,palladium, calcium or rare earth metals.

Generally, however, the zeolite either directly or via initial ammoniumexchange followed by calcination, is preferably hydrogen exchanged suchthat a predominate proportion of its exchangeable cations are hydrogenions. In general, it is contemplated that more than 50 percent andpreferably more than 75 percent of the cationic sites of the crystallinealuminosilicate zeolite will be occupied by hydrogen ions.

The above crystalline aluminosilicate zeolites employed are contactedwith a phosphorus compound. Representative phosphorus-containingcompounds include derivatives of groups represented by PX₃, RPX₂, R₂ PX,R₃ P, X₃ PO, (XO)₃ PO, (XO)₃ P, R₃ P=O, R₃ P=S, RPO₂, RPS₂, RP(O)(OX)₂,RP(S)(SX)₂, R₂ P(O)OX, R₂ P(S)SX, RP(OX)₂, RP(SX)₂, ROP(OX)₂, RSP(SX)₂,(RS)₂ PSP(SR)₂, and (RO)₂ POP(OR)₂, where R is an alkyl or aryl, such asa phenyl radical and X is hydrogen, R, or halide. These compoundsinclude primary, RPH₂, secondary, R₂ PH and tertiary, R₃ P, phosphinessuch as butyl phosphine; the tertiary phosphine oxides R₃ PO, such astributylphosphine oxide, the tertiary phosphine sulfides, R₃ PS, theprimary, RP(O)(OX)₂, and secondary, R₂ P(O)OX, phosphonic acids such asbenzene phosphonic acid; the corresponding sulfur derivatives such asRP(S)(SX)₂ and R₂ P(S)SX, the esters of the phosphonic acids such asdiethyl phosphonate, (RO)₂ P(O)H, dialkyl alkyl phosphonates, (RO)₂P(O)R, and alkyl dialkylphosphinates, (RO)P(0)R₂ ; phosphinous acids, R₂POX, such as diethylphosphinous acid, primary, (RO)P(OX)₂, secondary,(RO)₂ POX, and tertiary, (RO)₃ P, phosphites; and esters thereof such asthe monopropyl ester, alkyl dialkylphosphinites, (RO)PR₂, and dialkylalkylphosphonite, (RO)₂ PR esters. Corresponding sulfur derivatives mayalso be employed including (RS)₂ P(S)H, (RS)₂ P(S)R, (RS)P(S)R₂, R₂ PSX,(RS)P(SX)₂, (RS)₂ PSX, (RS)₃ P, (RS)PR₂ and (RS)₂ PR. Examples ofphosphite esters include trimethylphosphite, triethylphosphite,diisopropylphosphite, butylphosphite; and pyrophosphites such astetraethylpyrosphosphite. The alkyl groups in the mentioned compoundscontain one to four carbon atoms.

Other suitable phosphorus-containing compounds include the phosphorushalides such as phosphorus trichloride, bromide, and iodide, alkylphosphorodichloridites, (RO)PCl₂, dialkyl phosphorochloridites, (RO)₂PX, dialkylphosphionochloridites, R₂ PCl, alkylalkylphosphonochloridates, (RO)(R)P(O)Cl, dialkyl phosphinochloridates,R₂ P(O)Cl and RP(O)Cl₂. Applicable corresponding sulfur derivativesinclude (RS)PCl₂, (RS)₂ PX, (RS)(R)P(S)Cl and R₂ P(S)Cl.

Preferred phosphorus-containing compounds include diammonium hydrogenphosphate, ammonium dihydrogen phosphate, diphenyl phosphine chloride,trimethylphosphite and phosphorus trichloride, phosphoric acid, phenylphosphine oxychloride, trimethylphosphate, diphenyl phosphinous acid,diphenyl phosphinic acid, diethylchloro thiophosphate, methyl acidphosphate and other alcohol-P₂ O₅ reaction products.

Reaction of the zeolite with the phosphorus compound is effected bycontacting the zeolite with such compound. Where the treating phosphoruscompound is a liquid, such compound can be in solution in a solvent atthe time contact with the zeolite is effected. Any solvent relativelyinert with respect to the treating compound and the zeolite may beemployed. Suitable solvents include water and aliphatic, aromatic oralcoholic liquids. Where the phosphorus-containing compound is, forexample, trimethylphosphite or liquid phosphorus trichloride, ahydrocarbon solvent such as n-octane may be employed. Thephosphorus-containing compound may be used without a solvent, i.e., maybe used as a neat liquid. Where the phosphorus-containing compound is inthe gaseous phase, such as where gaseous phosphorus trichloride isemployed, the treating compound can be used by itself or can be used inadmixture with a gaseous diluent relatively inert to thephosphorus-containing compound and the zeolite such as air or nitrogenor with an organic solvent, such as octane or toluene.

Prior to reacting the zeolite with the phosphorus-containing compound,the zeolite may be dried. Drying can be effected in the presence of air.Elevated temperatures may be employed. However, the temperature shouldnot be such that the crystal structure of the zeolite is destroyed.

Heating of the phosphorus-containing catalyst subsequent to preparationand prior to use is also preferred The heating can be carried out in thepresence of oxygen, for example, air. Heating can be at a temperature ofabout 150° C. However, higher temperatures, i.e., up to about 500° C.are preferred. Heating is generally carried out for 1-5 hours but may beextended to 24 hours or longer. While heating temperatures above about500° C. can be employed, they are not necessary. At temperatures ofabout 1000° C., the crystal structure of the zeolite tends todeteriorate. After heating in air at elevated temperatures, phosphorusis present in oxide form.

The amount of phosphorus oxide incorporated with the zeolite should beat least about 0.25 percent by weight. However, it is preferred that theamount of phosphorus oxide in the zeolite be at least about 2 percent byweight, particularly when the same is combined with a binder, e.g. 35weight percent of alumina. The amount of phosphorus oxide can be as highas about 25 percent by weight or more depending on the amount and typeof binder present. Preferably, the amount of phosphorus oxide added tothe zeolite is between about 0.7 and about 15 percent by weight.

The amount of phosphorus oxide incorporated with the zeolite by reactionwith elemental phosphorus or phosphorus-containing compound will dependupon several factors. One of these is the reaction time, i.e., the timethat the zeolite and the phosphorus-containing source are maintained incontact with each other. With greater reaction times, all other factorsbeing equal, a greater amount of phosphorus is incorporated with thezeolite. Other factors upon which the amount of phosphorus incorporatedwith the zeolite is dependent include reaction temperatures,concentration of the treating compound in the reaction mixture, thedegree to which the zeolite has been dried prior to reaction with thephosphorus-containing compound, the conditions of drying of the zeoliteafter reaction of the zeolite with the treating compound, and the amountand type of binder incorporated with the zeolite.

The zeolite containing phosphorus oxide is then further combined withmagnesium oxide by contact with a suitable compound of magnesium.Representative magnesium-containing compounds include magnesium acetate,magnesium nitrate, magnesium benzoate, magnesium proprionate, magnesium2-ethylhexoate, magnesium carbonate, magnesium formate, magnesiumoxylate, magnesium amide, magnesium bromide, magnesium hydride,magnesium lactate, magnesium laurate, magnesium oleate, magnesiumpalmitate, magnesium silicylate, magnesium stearate and magnesiumsulfide.

Reaction of the zeolite with the treating magnesium compound is effectedby contacting the zeolite with such compound. Where the treatingcompound is a liquid, such compound can be in solution in a solvent atthe time contact with the zeolite is effected. Any solvent relativelyinert with respect to the treating magnesium compound and the zeolitemay be employed. Suitable solvents include water and aliphatic, aromaticor alcoholic liquid. The treating compound may also be used without asolvent, i.e. may be used as a neat liquid. Where the treating compoundis in the gaseous phase, it can be used by itself or can be used inadmixture with a gaseous diluent relatively inert to the treatingcompound and the zeolite such as helium or nitrogen or with an organicsolvent, such as octane or toluene.

Heating of the magnesium compound impregnated catalyst subsequent topreparation and prior to use is preferred. The heating can be carriedout in the presence of oxygen, for example, air. Heating can be at atemperature of about 150° C. However, higher temperatures, i.e. up toabout 500° C. are preferred. Heating is generally carried out for 1-5hours but may be extended to 24 hours or longer. While heatingtemperatures above about 500° C. may be employed, they are generally notnecessary. At temperatures of about 1000° C., the crystal structure ofthe zeolite tends to deteriorate. After heating in air at elevatedtemperatures, the oxide form of magnesium is present.

The amount of magnesium oxide incorporated in the calcined phosphorusoxide-containing zeolite should be at least about 0.25 percent byweight. However, it is preferred that the amount of magnesium oxide inthe zeolite be at least about 1 percent by weight, particularly, whenthe same is combined with a binder, e.g. 35 weight percent of alumina.The amount of magnesium oxide can be as high as about 25 percent byweight or more depending on the amount and type of binder present.Preferably, the amount of magnesium oxide added to the zeolite isbetween about 1 and about 15 percent by weight.

The amount of magnesium oxide incorporated with the zeolite by reactionwith the treating solution and subsequent calcination in air will dependon several factors. One of these is the reaction time, i.e. the timethat the zeolite and the magnesium-containing source are maintained incontact with each other. With greater reaction times, all other factorsbeing equal, a greater amount of magnesium oxide is incorporated withthe zeolite. Other factors upon which the amount of magnesium oxideincorporated with the zeolite is dependent include reaction temperature,concentration of the treating compound in the reaction mixture, thedegree to which the zeolite has been dried prior to reaction with thetreating compound, the conditions of drying of the zeolite afterreaction of the zeolite with the magnesium compound and the amount andtype of binder incorporated with the zeolite.

After contact of the phosphorus oxide-containing zeolite with themagnesium reagent, the resulting composite is dried and heated in amanner similar to that used in preparing the phosphorus oxide-containingzeolite.

In practicing the desired methylation process, it may be desirable toincorporate the modified zeolite in another material resistant to thetemperatures and other conditions employed in the methylation process.Such matrix materials include synthetic or naturally occurringsubstances as well as inorganic materials such as clay, silica and/ormetal oxides. The latter may be either naturally occurring or in theform of gelatinous precipitates or gels including mixtures of silica andmetal oxides. Naturally occurring clays which can be composited with themodified zeolite include those of montmorillonite and kaolin families,which families include the sub-bentonites and the kaolins commonly knownas Dixie. McNamee-Georgia and Florida clays or others in which the mainmineral constitutent is halloysite, kaolinite, dickite, nacrite oranauxite. Such clays can be used in the raw state as orignally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the modified zeolites employedherein may be composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-berylia, silica-titania as well as ternary compositions, such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina magnesiaand silica-magnesia-zirconia. The matrix may be in the form of a cogel.The relative proportions of finely divided modified zeolite an inorganicoxide gel matrix may vary widely with the zeolite content ranging frombetween about 1 to 99 percent by weight and more usually in the range ofabout 5 to about 80 percent by weight of the composite.

Methylation of toluene in the presence of the above described catalystis effected by contact of the toluene with a methylating agent,preferably methanol, at a temperature between about 250° C. and about750° C. and preferably between about 500° C. and about 700° C. At thehigher temperatures, the zeolites of high silica/alumina ratio arepreferred. For example, ZSM-5 of 300 SiO₂ /Al₂ O₃ ratio and upwards isvery stable at high temperatures. The reaction generally takes place atatmospheric pressure, but the pressure may be within the approximaterange of 1 atmosphere to 1000 psig. The molar ratio of methylating agentto toluene is generally between about 0.05 and about 5. When methanol isemployed as the methylating agent a suitable molar ratio of methanol totoluene has been found to be approximately 0.1-2 moles of methanol permole of toluene. With the use of other methylating agents, such asmethylchloride, methylbromide, dimethylether, methyl carbonate, lightolefins or dimethylsulfide, the molar ratio of methylating agent totoluene may vary within the aforenoted range. Reaction is suitablyaccomplished utilizing a weight hourly space velocity of between about 1and about 2000 and preferably between about 5 and about 1500. Thereaction product consisting predominantly of para-xylene or a mixture ofpara- and ortho-xylene together with comparatively smaller amounts ofmeta-xylene may be separated by any suitable means, such as by passingthe same through a water condenser and subsequently passing the organicphase through a column in which chromatographic separation of the xyleneisomers is accomplished.

The process of this invention may be carried out as a batch-type,semi-continuous or continuous operation utilizing a fixed or moving bedcatalyst system. A preferred embodiment entails use of a fluidizedcatalyst zone wherein the reactants, i.e., toluene and methylatingagent, are passed concurrently or countercurrently through a movingfluidized bed of the catalyst. The fluidized catalyst after use isconducted in an oxygen-containing atmosphere, e.g., air, at an elevatedtemperature, after which the regenerated catalyst is recycled to theconversion zone for further contact with the toluene and methylatingagent reactants.

The following examples will serve to illustrate the process of theinvention without limiting the same.

EXAMPLE 1

A catalyst containing 65 weight percent acid ZSM-5 and 35 weight percentalumina was prepared as follows:

A sodium silicate solution was prepared by mixing 8440 lb. of sodiumsilicate (Q Brand--28.9 weight percent SiO₂, 8.9 weight percent Na₂ Oand 62.2 weight percent H₂ O) and 586 gallons of water. Afer addition of24 lb. of a dispersant of a sodium salt of polymerized substitutedbenzenoid alkyl sulfonic acid combined with an inert inorganicsuspending agent (Daxad 27), the solution was cooled to approximately55° F. An acid alum solution was prepared by dissolving 305 lb. aluminumsulfate (17.2 Al₂ O₃), 733 lb. sulfuric acid (93%) and 377 lb. sodiumchloride in 602 gallons of water. The solutions were gelled in a mixingnozzle and discharged into a stirred autoclave. During this mixingoperation, 1200 lb. of sodium chloride was added to the gel andthoroughly mixed in the vessel. The resulting gel was thoroughlyagitated and heated to 200° F. in the closed vessel. After reducingagitation, an organic solution prepared by mixing 568 lb.tri-n-propylamine, 488 lb. n-propyl bromide and 940 lb. methyl ethylketone was added to the gel. This mixture was reacted for 14 hours at atemperature of 200°-210° F. At the end of this period, agitation wasincreased and these conditions maintained until the crystallinity of theproduct reached at least 65% ZSM-5 as determined by X-ray diffraction.Temperature was then increased to 320° F. until crystallization wascomplete. The residual organics were flashed from the autoclave and theproduct slurry was cooled.

The product was washed by decantation using a flocculant of polyammoniumbisulfate. The washed product containing less than 1% sodium wasfiltered and dried. The weight of dried zeolite was approximately 2300lb.

The dried product was mixed with alpha alumina monohydrate and water(65% zeolite, 35% alumina binder on ignited basis) then extruded to formof 1/16 inch pellet with particle density <0.98 gram/cc and crushstrength of >20 lb./linear inch.

After drying, the extruded pellets were calcined in nitrogen (700-1000SCFM) for 3 hours at 1000° F., cooled and ambient air was passed throughthe bed for 5 hours. The pellets were then ammonium exchanged for onehour at ambient temperature (240 lb. ammonium nitrate dissolved inapproximately 800 gallons of deionized water). The exchange was repeatedand the pellets washed and dried. Sodium level in the exchanged pelletswas less than 0.05 weight percent.

The dried pellets were calcined in a nitrogen-air mixture (10-12.5%air-90-87.5% nitrogen) for 6 hours at 1000° F. and cooled in nitrogenalone.

EXAMPLE 2

To a solution of 7 grams of 85% H₃ PO₄ in 10 ml. of water was added 10grams of HZSM-5 extrudate prepared as in Example 1. The extrudate waspermitted to remain in such solution at room temperature overnight.After filtration and drying at 120° C. for 3 hours, it was calcined at500° C. for 3 hours to give 11.5 grams of phosphorus-modified ZSM-5.

Ten grams of the above phosphorus-modified ZSM-5 was then added to asolution of 25 grams of magnesium acetate tetrahydrate in 20 ml. ofwater which was permitted to stand at room temperature overnight. Afterfiltration and drying at 120° C., it was calcined at 500° C. for 3 hoursto give 10.9 grams of magnesium-phosphorus-modified ZSM-5. Analysisshowed the modifier concentration to be 7.4 weight percent phosphorusand 4.2 weight percent magnesium.

EXAMPLE 3

A mixture of toluene and methanol wherein the molar ratio of toluene tomethanol was 4 was passed over the catalyst of Example 2 at a weighthourly space velocity of 10.5 (based on total catalyst) at 450° C.Conversion of toluene was 8.5 percent and the concentration ofpara-xylene in total xylenes was 98.5 percent.

EXAMPLES 4-7

A toluene-methanol mixture having a molar ratio of toluene to methanolof 4 was passed over the catalyst of Example 2 at a weight hourly spacevelocity of 3.4 under varying temperature conditions set forth belowwith the resulting toluene conversion and para-xylene production.

    ______________________________________                                                                       Para-Xylene                                             Temp.     % Toluene   Concentration In                               Example  °C.                                                                              Conversion  Total Xylenes                                  ______________________________________                                        4        450       11.2        97.4                                           5        500       14.8        97.2                                           6        550       19.6        96.5                                           7        600       23.5        93.4                                           ______________________________________                                    

EXAMPLES 8-11

A phosphorus-magnesium-modified ZSM-5 catalyst prepared in a mannersimilar to that of Example 2 but containing 2.9 weight percentphosphorus and 4.9 weight percent magnesium was employed for alkylationof toluene with methanol.

A feed mixture wherein the molar ratio of toluene to methanol was 4 waspassed over the above catalyst at a weight hourly space velocity of 10under varying temperature conditions set forth below with the resultingtoluene conversion and para-xylene production.

    ______________________________________                                                                       Para-Xylene                                             Temp.     % Toluene   Concentration In                               Example  °C.                                                                              Conversion  Total Xylenes                                  ______________________________________                                        8        400       9.4         98.3                                           9        450       11.0        97.1                                           10       500       14.4        96.0                                           11       550       18.8        94.1                                           ______________________________________                                    

It will be seen from the foregoing examples that in every instanceutilizing the phosphorus-magnesium-modified ZSM-5 catalyst, methylationof toluene afforded a very marked increased in selectivity forpara-xylene production over the concentration of this component, i.e.,about 24 weight percent, in the normal equilibrium mixture of xyleneisomers.

What is claimed is:
 1. A process for the selective production ofpara-xylene which comprises reacting toluene with a methylating agentunder methylation conditions in the presence of a catalyst comprising acrystalline aluminosilicate zeolite, said zeolite having a silica toalumina ratio of at least about 12, a constraint index within theapproximate range of 1 to 12, said catalyst having been modified by theaddition thereto of phosphorus oxide and magnesium oxide as a result ofincorporation of a compound of phosphorus and a compound of magnesiumwith said zeolite, each of said oxides being incorporated byimpregnation of said zeolite therewith and being present in an amount ofat least about 0.25 percent by weight.
 2. The process of claim 1 whereinsaid crystalline aluminosilicate zeolite is characterized by asilica/alumina ratio in excess of
 30. 3. The process of claim 1 whereinsaid methylating agent is methanol, methyl chloride, methyl bromide,dimethylether or dimethylsulfide.
 4. The process of claim 1 wherein saidcrystalline aluminosilicate is ZSM-5.
 5. The process of claim 1 whereinsaid methylation conditions include a temperature between about 250° C.and about 750° C., a pressure within the approximate range of 1atmosphere to 1000 psig, a weight hourly space velocity between about 1and about 2000 and a molar ratio of methylating agent to toluene betweenabout 0.05 to about
 5. 6. The process of claim 4 wherein saidmethylating agent is methanol.
 7. The process of claim 1 whereinphosphorus oxide and magnesium oxide are each present in an amount ofbetween about 0.25 and about 25 weight percent.
 8. The process of claim1 wherein said phosphorus oxide is present in an amount of between about0.7 and about 15 weight percent and magnesium oxide is present in anamount of between about 1 and about 15 weight percent.
 9. The process ofclaim 1 wherein the crystalline aluminosilicate zeolite is combined inan amount between about 1 and about 90 weight percent in a bindertherefor.
 10. The process of claim 1 wherein said binder is alumina. 11.The process of claim 1 wherein the crystalline aluminosilicate zeoliteis predominantely in the hydrogen form.